No. 28. Bays and Estuaries

Landscape Setting. This habitat is adjacent to Westside Riparian-Wetlands, Coastal Dunes and Beaches, Westside Lowland Conifer-Hardwood Forest, Coastal Headlands and Islets, Marine Nearshore, and Inland Marine Deeper Waters habitats. Major uses of bays and estuaries are recreation, tourism, the shellfish industry, and navigation. The terrestrial interface portions of this habitat have been extensively converted for agricultural crop production, livestock grazing, and residential and commercial development. Water channels of many areas have been dredged for ship navigation.

Structure. At the most seaward extent (e.g., river mouths), water depths are shallow (mostly <20 ft [6 m]) except for dredged channels. This habitat is strongly influenced by the daily tides and currents. Depending on location, mean higher high water to mean lower low water ranges from 6.1 to 10.2 ft (1.9 to 3.1 m). Tidal currents in channels of the principal estuaries typically range from 1 to 5 knots (0.5 to 2.6 m/sec) ¹⁷⁶.

Diverse habitats result from riverine discharges and tidal fluxes, salinity, mixing, sedimentation, discharge, and insolation. Unconsolidated or consolidated tideflats are composed of rocks, gravel, sand, silt and clay as well as abundant organic material ^{68, 204}. Inundated by daily tidal flows, tideflats may support eelgrass, various algal species, and invertebrate communities ⁵. Eelgrass meadows create protected environments and structured habitats for various wildlife species ¹⁷³. Salt marshes form at the upper tidal boundary above tideflats ¹²⁰. Salt marshes are usually open to closed graminoid or forb communities. Highly branched estuarine channels drain across salt marshes and tideflats, creating a diverse mix of structures ¹⁹². At the most inland extent of this habitat, transitional marsh forms between salt marshes and bordering upland vegetation dominated by grass or woody vegetation ⁶.

The Columbia River estuary is characterized as a partially mixed estuary and can be divided into 3 sections along the salinity gradient: from the mouth to about river mile 7 it is basically marine; from river mile 7 to mile 23 it is transitional (mixing); and above river mile 23 it is fluvial (fresh water). Pruter and Alverson ¹⁷⁹ compiled available physical and biological studies at the interface between riverine and marine waters in the nearshore aspects of the Columbia River estuary and adjacent waters.

Composition. Eelgrass meadows stabilize submerged tideflats and are co-dominated by surfgrass and eelgrass species. Three diagnostic surfgrass species (Phyllospadix scouleri, P. torreyi, and P. serrulatus) occur on rocky substrates in exposed waters, whereas 2 species of eelgrasses (Zostera marina, Z. japonica) are characteristic of mud or mixed mud-sand substrates in areas sheltered from turbulent waters ^{68, 173}. Highly productive macroalgae that dominate estuarine channels include various blue-green algae, green algae (Enteromorpha spp.) and rockweed (Fucus spp.) ¹⁹². Tideflats bordering salt marshes often are co-dominated by pickleweed (Salicornia virginica), arrowgrass (Triglochin maritima) and three-square rush (Scirpus americanus) ⁵. The transition to higher areas of the low-marsh zone is indicated by the dominance of jaumea (Jaumea carnosa), saltgrass (Distichlis spicata), and Lyngby’s sedge (Carex lyngbyei) ⁶⁸. Major components of mid- and high salt marsh areas are alkaligrass (Puccinellia pumila) and Canadian sand spurry (Spergularia canadensis) ¹²⁰. Salt rush (Juncus lesueurii), tufted hairgrass (Deschampsia caespitosa), Pacific silverweed (Argentina egedii) and spreading bentgrass (Agrostis stolonifera) are salt-tolerant upland species diagnostic of high salt marshes that experience freshwater runoff or riverine discharge ⁶.

Other Classifications and Key References. Cowardin et al. ⁵³ included marine and estuarine systems of the Columbian Province. Dethier ⁶⁸ described a classification for marine and estuarine habitat types in Washington State. Habitat types are defined by depth, substratum type, energy level, and a few modifiers. Species (plants and animals) are described for combinations of these physical variables. The Oregon Gap II Project ¹²⁶ and Oregon Vegetation Landscape-Level Cover Types ¹²⁷ that would represent this type are exposed tidal flats and estuarine emergent. Harper et al. ¹⁰⁶ described a shore-zone sensitivity mapping system. Proctor et al. ¹⁷⁶ described an ecological characterization of the Pacific Northwest Coastal Region, including physical and chemical environments as well as socioeconomic aspects of watersheds of the region. Schoch and Dethier ¹⁸⁹ provided high-resolution data on the physical features and associated biota of Puget Sound’s shorelines using the SCALE model (Shoreline Classification and Landscape Extrapolation). Downing ⁷⁶ offered a detailed review of the geological and broad ecological development of Puget Sound.

Natural Disturbance Regime. Natural disturbance perpetuates the dynamic, transitional nature of this habitat. Tides, seasonal riverine discharges, winds, storm events, erosion, and accretion are the primary natural processes that shape this habitat. Tides are mixed, characterized by 2 unequal high and low tides daily, with varying intrusion into estuaries and bays at different locations along the coast. Tides and winds push salt-water wedges up through the system, causing varying degrees of mixing with incoming riverine waters and significant vertical stratification ⁵. Riverine discharges and fresh-water runoff vary seasonally with precipitation and freshet regimes. Generally, a large range in annual discharge exists with high volumes of fresh water entering the system in winter and significantly reduced flows in summer ¹⁹². Short-term storm events produce dramatic variations in physical habitat conditions. Sudden erosion or accretion may result from strong oceanic currents at the mouth of the system or from increased fresh water discharges at the head of the system. For a detailed conceptual model of disturbance regimes in estuary zones, see Proctor et al. ¹⁷⁶.

Succession and Stand Dynamics. General successional stages reflect unconsolidated barren tideflats to stabilized high salt marshes and salt meadows. Unvegetated tideflats are colonized by pioneer plants, commonly eelgrass, that are tolerant of extended tidal inundation and vary depending on sediment type ¹⁷³. Initial colonization causes sediment accretion and gradual rise in land elevation, changes that shift environmental conditions and permit other plants to establish. Arrowgrass, pickleweed, sand spurry, and spike rush can invade the emerging marsh, further increasing and stabilizing substrates. Saltgrass and sedge establish on higher areas of the marsh. When initial colonizers die back, tufted hairgrass and salt rush may establish ⁵. Various exotic species have become naturalized in Oregon and Washington, including spreading bentgrass (Agrostis stolonifera) and sand spurry (Spergularia marina) introduced from Europe, brass buttons (Cotula coronopifolia) introduced from South Africa, and marsh cordgrass (Spartina alterniflora) introduced from the Atlantic Coast of North America ^{6, 120}. These successional stages can be disrupted by riverine or tidal scouring and succession can be reinitiated at any point.

Effects of Management and Anthropogenic Impacts. Management, water quality, contaminants, and land-use practices have altered significant portions of this habitat and continue to impact remaining areas ²¹⁶. The dredging and filling of marshes and tideflats to serve various human needs remove estuarine vegetation. Channel flow, tidal inundation, and fresh water discharges are disrupted by construction of seawalls, jetties, dikes, and dams. The physical and chemical conditions of these habitats are degraded by the discharge of municipal, industrial, and agricultural effluents. Functional plant and animal communities are altered by domestic and agricultural runoff of pesticides, herbicides, and fertilizers. Invasions of exotic plants (e.g., Spartina) and invertebrates (e.g., green crabs) pose significant, long-term ecological and economic threats to this habitat. Large tracts of habitat have been lost and converted for coastal development. Additionally, upland activities occurring throughout the watershed, including logging, mining, and hydroelectric power development, can have destructive impacts downstream in estuarine and bay environments ^{192, 205}.

Status and Trends. Significant quantitative and qualitative alterations of this habitat have occurred with Euro-American settlement. Although natural erosion and accretion processes continue, most habitat modification can be attributed to anthropogenic impacts ¹⁹². Because of original diking for crop production and flood control, almost no areas of natural high marsh remain in Oregon ¹²⁰. These dikes, and other more recent barriers, prevent natural recovery and re-establishment of this habitat. Remaining examples of the bay and estuarine habitat exist in various conditions, from the more natural areas, areas undergoing active restoration, to the more prevalent polluted, degraded, or overused areas throughout Oregon and Washington. With increasing population pressures in coastal areas and the corresponding threats of habitat use and conversion, future trends will likely be continued degradation and reduction of remaining bay and estuarine areas.